As geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. In optical metrology the reflected energy that results when an optical beam is directed at a sample can be analyzed using a range of different optical techniques. One known optical metrology technique is Modulated Optical Reflectance (MOR) sometimes also called photo-modulated reflectance (PMR). In this technique a pump laser is switched on and off to create an intensity-modulated pump beam. The pump beam is projected against the surface of a sample causing localized heating of the sample. As the pump laser is modulated, the localized heating (and subsequent cooling) creates a train of thermal and plasma waves which reflect and scatter off various features and interact with various regions within the sample to alter the flow of heat and/or plasma from the pump beam spot. These thermal and plasma waves have a direct effect on the surface reflectivity of the sample. Features and regions below the sample surface that alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
PhotoModulated Reflectance (PMR) type systems use external means to induce thermal or plasma waves in the sample under study. PMR-type systems, used to study a range of attributes including material composition and layer thickness, are described in more detail in U.S. Pat. Nos. 4,634,290; 4,646,088; 4,679,946; 4,854,710; 5,854,719; 5,978,074; 5,074,669; and 6,452,685, each of which is incorporated by reference herein.
PMR-type systems can be used for the measurement and analysis of the dopants added to semiconductor wafers. Dopants are ions that are implanted to semiconductors during a process known as ion implantation. Ion implantation damages the crystal lattice as incoming ions come to rest. This damage is typically proportional to the concentration and depth of ions within the crystal lattice. This makes measurement of damage an effective substitute for direct measurement of dopant concentration and depth. PMR-type systems have proven to be adept at measuring damage and have been widely used for post implantation evaluation.
The relationship between dopant concentration and the PMR amplitude is monotonic for low dopant concentrations. As dopant concentration increases the monotonic relationship breaks down. At high concentrations, the PMR amplitude behavior sometimes is no longer monotonic and as a result cannot be used to accurately derive corresponding dopant concentrations. The same sort of breakdown may occur as implanted ions become heavier (e.g., As+ or P+ ions). In both cases, this is attributable to the formation of a Si amorphous layer resulting in modulated interference effects. PMR phase information becomes flat or insensitive to changes in concentration at high dopant concentrations or where heavy ions are implanted.
Measuring the DC reflectivity of both the pump and probe beams in addition to the modulated optical reflectivity signal carried on the probe beam, i.e., using the DC reflectivity data at two wavelengths often resolves some ambiguities in the measurement caused, for example by the presence of the Si oxide layer on the surface of the sample. See U.S. Pat. No. 5,074,669 (incorporated in this document by reference).
A method to provide this type of reliable and effective measurement capability for high dopant concentrations and ion implantation of relatively heavy ions is shown in U.S. Pat. Nos. 6,989,899 and 7,002,690, both of which have been incorporated herein by reference.
This prior art method simultaneously monitors ion implantation dose, damage and/or dopant depth profiles in ion-implanted semiconductors. The measurement method is logically divided into two steps: a calibration and a measurement step. During the calibration step, the photo-modulated reflectance of a known damage profile is characterized, using a PMR-type optical metrology tool to record both quadrature (Q) and in-phase (I) values for a series of specially prepared calibration subjects. Each calibration subject is fabricated at the same implantation energy. As a result, variations recorded by the system are largely attributable to variations in dopant concentration.
In the measurement step, I-Q measurements for a test subject are obtained empirically. A calibration line is defined within an I-Q plane. The slope of the calibration line is defined by the implantation energy used to create the calibration subject. The calibration line is used to define a calibration region within the I-Q plane, by defining an upper boundary line that has a slightly greater slope than the calibration line and a lower boundary line that has a slightly smaller slope than the calibration line. The calibration region is the area between the upper and lower boundary lines and includes all points within a specified distance (often defined in terms of a percentage) of the calibration line. For each subject wafer, the PMR-type system makes one or more measurements. Measurements that fall within the calibration region are known to have the damage profile close to that of the calibration subject.
The empirically obtained I-Q measurements are then compared to determine if they fall within the identified calibration region of I-Q space. This comparison indicates whether the test subject has a damage profile that is similar to the known damage profile. Measurements that do not fall within this region are assumed to deviate from the known damage profile of the calibration subject. Details of the method are set forth in the aforementioned U.S. Pat. Nos. 6,989,899 and 7,002,690.
While MOR can provide an effective method of accepting or rejecting wafers that provide acceptable accuracy even when dopant concentrations are high or where heavy ions have been implanted, it does not attempt to address the nature and character of those measurements which fall outside of the calibration region.
Prior art systems for ion implant metrology such as MOR are capable of detecting faults by measuring the MOR signal magnitude at a given time interval. However, no identification of faults have been performed on the system itself, leaving it up to the operator to determine off-line what kind of implanter malfunction caused each particular fault. In addition, prior art systems use only the MOR signal to monitor implanters for potential faults. Therefore prior art systems are unable to identify the cause of faults of certain types, e.g., faults caused by the wrong energy, the wrong dose, the wrong species, etc.
It is within this context that embodiments of the present invention arise.